The present invention relates to a method and apparatus for safely controlling the undesired reactions in black liquor recovery boilers. More specifically, the invention relates to the control of explosive reactions in black liquor recovery boilers, caused principally by the escape or leakage of water into contact with the smelt product which may accumulate in the furnace of the recovery boiler.
Prior art patents which have focused on this problem have suggested various solutions for preventing explosive reactions in black liquor recovery boilers. For example, U.S. Pat. No. 3,447,895, issued June 3, 1969 teaches a method of preventing explosions by introducing onto the smelt in the furnace an aqueous quenching solution to rapidly cool the smelt bed to temperatures below the explosive range. U.S. Pat. No. 3,615,175, issued Oct. 26, 1971 teaches the introduction of a solid compound capable of highly endothermic chemical reaction upon thermal decomposition in the furnace. The decomposition reaction serves to inert the furnace with a non-flammable gas produced, thereby eliminating further production in the recovery process and solidifying the molten smelt. U.S. Pat. No. 4,106,978, issued Aug. 15, 1978 discloses the addition of a porous, high surface area powder which is coated with an anti-wetting agent. U.S. Pat. No. 3,403,642, issued Oct. 1, 1968 discloses an apparatus for increasing the temperature of the char bed upon detection of excessive water leakage into the furnace, to consume the char bed as quickly as possible while keeping furnace temperatures high enough to consume hydrogen which may be produced at this time.
Black liquor recovery boilers are critical elements in the implementation of an industrial process known as the Kraft pulping process which is used in many paper mills in the United States and throughout the world. The Kraft process is a closed-loop chemical process for producing long-fiber pulp used for making high strength paper from a variety of organic materials, usually wood. In the implementation of this process, wood logs are typically debarked and chipped into very small pieces, and fed into a digester where they are cooked in a solution of sodium hydroxide and sodium sulfide. This solution is known as "white liquor", and steam is added to the process for a number of hours to help dissolve the lignin binder which holds the wood fibers together.
After the cooking step has been performed the cellulose fibers are separated from the remaining solution, which is called "black liquor", and are further prepared for use and manufacture into paper. The dilute black liquor is usually run through one or more evaporation steps to increase the solids concentration to 62-65 percent, which is necessary for combustion.
The black liquor is ultimately heated and pressurized, typically to 220.degree. F.-240.degree. F. at 15-30 psig, and is fed through spray nozzles projecting into the black liquor furnace. The nozzles produce rather coarse droplets which are sprayed into the furnace interior and allowed to drop downwardly under the influence of gravity. Inside the furnace, a flame burns which acts upon the droplets as they fall toward the bed of the furnace. At boiler startup, the flame is supplied by auxiliary burners, but the black liquor combustion eventually becomes self-sustaining and the auxiliary burners are shut off. Burning of the black liquor accomplishes an evaporation of water from the droplets as they fall, and a partial combustion of the liquor solids within the droplets themselves.
This process produces significant amounts of heat, which is recovered in the furnace through the use of pipes running through the furnace. The inorganic constituents which remain from the burning process drop to the bed of the furnace, into what is known as a "char bed." Continued combustion in the char bed produces a reducing atmosphere which converts sodium sulfate to sodium sulfide, one of the chemicals desired for re-use in the Kraft process. The molten chemicals in the char bed are referred to as "smelt", a chief constituent in the smelt being sodium sulfide, which occurs as a result of the reduction of the sodium sulfate constituents sprayed into the furnace. This material is permitted to continuously drain into a dissolving tank where it is quenched and dissolved in a weak wash to form what is known as "green liquor".
The chemical recovery process in the black liquor recovery boiler is both efficient and safe as long as the furnace and all related equipment is physically sound, and as long as the black liquor emitted from the spray nozzles has been properly concentrated and is capable of sustained burning as it leaves the nozzles. However, should water come into contact with the smelt accumulated on the furnace floor of the recovery boiler, violent explosions can occur. The history of black liquor recovery boilers is replete with such incidents, and considerable time and effort has been expended toward understanding the exact cause of these explosions and devising methods to prevent them from occurring. Several principal theories have evolved, and two are described in the various prior art patents cited herein. One theory holds that the explosions are physical in nature, resulting from "encapsulation" of water in the smelt bed. Another theory holds that the explosions are chemical in nature, resulting from the evolution of explosive gases when moisture comes in contact with the smelt. The cited patent references teach various methods and systems for preventing smelt/water explosions, but the problem still exists in the pulp and paper industry. More recently, the theory centering on explosions resulting from interface of two or more liquids having significantly different temperatures has been applied to explain the smelt/water interface. It is theorized that water in contact with molten smelt first dissolves a quantity of smelt, forming a solution known as green liquor. This green liquor has a superheat temperature much higher than that of water, thus accommodating the requirements for liquid/liquid interface explosions.
The sodium sulfide which is produced as a result of the burning process is highly reactive with water, and upon contact with moisture forms hydrogen sulfide, a highly toxic and combustible gas. The recovery furnace itself contains water and steam pipes used for the collection of heat, and further contains water cooled drainage pipes for draining the smelt from the furnace, all of which can create an extremely dangerous environment if leakage should occur. Further, the burning process is initially ignited by means of externally fueled burners, but once the process begins it is self sustaining through the burning of the black liquor which is sprayed into the furnace. If the flame goes out during this burning process the unburned black liquor will settle into the smelt at the bottom of the furnace, bringing with it quantities of unevaporated water which may quickly accumulate in the smelt. As noted above, this will result in the release of hydrogen sulfide gas which could produce an explosion. However, the prior art patents suggest that the smelt/water explosions will occur even when sodium sulfide is not present. Since the other chemicals in the smelt, principally sodium sulfate and sodium carbonate and perhaps sodium sulfite, do not evolve explosive or flammable gases when in contact with moisture, it is therefore evident that factors other than flammable gas reactions are involved. On the other hand, the teachings of the prior art indicate that there is more difficulty in controlling explosions that occur in smelts having high sulfide content, which suggests that something more is involved than the physical "encapsulation" of water, as discussed above.
Explosions in black liquor furnaces may be very violent, and can cause much destruction and even loss of life. Therefore, it is important to develop techniques and apparatus for responding to conditions within the black liquor furnace which, if uncontrolled, will result in one or more explosions. Further, it is desirable to initiate corrective process steps upon the detection of a hazardous condition to neutralize the chemical occurring within the smelt, to prevent the cumulative buildup of reactive agents and thereby to prevent any explosion.
The continued existence of explosion problems in black liquor recovery demonstrates that the prior art inventions are either not effective or are not sufficiently practical in application, or a combination of both, and improvements in the art are necessary and desirable.